Departure date 31 March, landing in Thailand on 1 April.. based on the information in the screenshot, and my own assumptions, no RT-PCR test necessary before departure in this scenario. But I will take no chances and proceed with my test before departure anyway. Better safe than sorry.

Thank you for all the answers...

In these uncertain times I will purchase flexible flight tickets for a number of reasons. A cheap bastard like myself normally skip this option 😀

I will initially book my return flight within these first 60 days. Then after >30 somewhat days in the country I will extend my visa with additional 30 days and then rebook my return flight to the end of my 90-day trip.

I have been traveling to Thailand twice a year now since 2008, but only on visa exempt entries, so this will be my first time with a TR visa. If I have understood correctly, I can do this 30-day extension at ANY immigration office in the country? Along with the necessary paperwork (my TM6 card, a TM30 receipt), one passport-size photo and fill in the TM7 form.